1. Field of the Invention
The present invention relates to a variable amplifier for amplifying a Radio Frequency (RF) signal in a wireless transmitter. More particularly, the present invention relates to an apparatus and a method for exponentially controlling a gain of a driver amplifier to drive a power amplifier which provides a signal to an antenna stage.
2. Description of the Related Art
A general wireless transmitter uses a power amplifier for emitting an RF signal via an antenna. For example, the power amplifier is disposed at the end of a receiver and sends a signal with high power via the antenna.
However, such an amplifier structure is unable to obtain both a significant gain and a significant increase in power, and exhibits an increased performance only when focusing on either the gain or the power. More particularly, it is difficult for the power amplifier to produce a high power and satisfy a high gain at the same time. Yet, both high power and high gain are necessary for characteristics of the transmitter.
Thus, a separate high-gain amplifier is used in a front end to drive the power amplifier. An amplifier at the end of the transmitter combines a Driver Amplifier (DA) and a Power Amplifier (PA).
Typically, a structure of the DA is a set of cascode amplifiers to attain a variable gain. A cascode structure has good input/output isolation, high output impedance, and high gain. For example, for a variable gain interval of 6 dB, a unit cascode amplifier includes eight sets which increase by multiples of two, for example, 1, 2, 4, 8, 16, 32, 64, and 128, and only the cascode amplifier corresponding to the intended gain operates among the cascode amplifier sets.
However, eight digital switches are required to control six gain modes, which adopt a 3-to-8 decoder. Since only one cascode amplifier operates and other cascode amplifier sets do not operate in one mode, this process is inefficient. For example, the area 256 times the smallest unit cascode amplifier size is required for the 8-step gain control, which increases a unit cost.
In addition, scalability is greatly degraded. In some cases, a structure of the wireless transmitter may need more various gains. The control based on 3 dB or 1 dB, rather than 6 dB, is quite complicated and feasible only based on an approximate value, and thus its accuracy also degrades. When the control mode uses 16 steps or 32 steps, not 8 steps, a control switch exponentially increases as such and the area is two times the branch of a maximum gain. As a result, the inefficiency still remains.
Therefore, a need exists for an apparatus and a method for exponentially controlling a gain of a driver amplifier to drive a power amplifier which provides a signal to an antenna stage.
The above information is presented as background information only to assist with an understanding of the present disclosure. No determination has been made, and no assertion is made, as to whether any of the above might be applicable as prior art with regard to the present invention.